Power amplifier circuits are used for amplifying radio frequency (RF) signals for transmission by a communication device. In operation, a power amplifier circuit receives an RF signal in the transmit path of the communication device, amplifies the RF signal, and provides the amplified signal to an antenna. To meet system requirements, the RF antenna power output must be maintained substantially constant. For example, existing standards for mobile cellular telephones specify 600 mW RF power output at the antenna.
Gallium arsenide (GaAs) MESFET (metal epitaxial semiconductor field effect transistor) devices are typically used in the amplification stage of power amplifier design. Such devices provide good performance across a wide frequency range, including the range from 1800 to 2000 MHZ. MESFET devices operated in depletion mode typically require a gate to source voltage (V.sub.gs) less than zero volts to turn off the transistors. A bias voltage is supplied to the MESFET to establish the V.sub.gs.
The threshold or pinch off voltage (V.sub.th) of a depletion mode MESFET has a negative temperature coefficient. That is, the threshold voltage becomes more negative with increasing temperature. When such a device is operating near pinch off, an increasingly more negative gate voltage will maintain a constant drain current as temperature increases. Constant drain current with respect to temperature results in lower RF gain of the amplification stage and lower power output, with increasing temperature. This is due to the MESFET transconductance, which has a negative temperature coefficient, and output conductance, which has a positive temperature coefficient. As a result, RF GaAs MESFET devices require a bias current which increases with increasing temperature to maintain constant gain and output power.
Conventional communications devices have used a closed loop feedback system for maintaining constant output power. In such a system, output power is detected at the antenna and a signal indicative of the output power is conveyed to a control circuit, such as a microprocessor, in the communication device. The controller than adjusts the bias current as required to maintain constant output power. Such conventional designs, however, significantly limit the performance of the power amplifier. For instance, a directional coupler is required for detecting output power at the antenna. The directional coupler causes excess power loss at the antenna. At maximum transmit power, the power loss could be as high as 1 dB or 25 percent. In addition, the circuit elements employed for detecting output power and adjusting bias current are not able to develop the negative (below 2 volts) gate to source voltage for biasing the depletion mode MESFET.
U.S. Pat. No. 5,724,004 entitled VOLTAGE BIAS AND TEMPERATURE COMPENSATION CIRCUIT FOR RADIO FREQUENCY POWER AMPLIFIER issued to Reif et al. on Mar. 3, 1998 attempts to overcome the aforementioned problems and discloses a power amplifier and bias circuit where the power amplifier includes a depletion mode MESFET for power amplification. The MESFET is biased via the bias circuit which includes a level shifter for providing the necessary gate to source voltage to the MESFET. The bias circuit output voltage varies over temperature to track the temperature variation of the MESFET in order to maintain a substantially constant RF output power.
Other implementations of temperature compensation circuits for power amplifiers typically utilize a gallium arsenide based differential input amplifier followed by a voltage level shifting buffer biased via current sources. However, current biasing causes the differential amplifier and level shifting circuitry to be less sensitive to bias voltage and temperature fluctuations. Therefore, temperature compensating control circuits currently in existence have a limited voltage control range, while temperature compensation occurs over a similarly limited temperature range. That is, current control circuits are operable to adjust only a certain amount of compensation to the power amplifier over a temperature range before requiring a complete termination of shut off or the power amplifier. Accordingly, a device that is both temperature and process sensitive and which performs level shifting and temperature compensation over a greater range of values is highly desired.